A twin-roll type continuous casting apparatus has been known as an apparatus for continuously casting a thin metallic plate. This twin-roll type continuous casting apparatus is, as described later with reference to the drawings, provided with a pair of mold rolls (hereinafter referred to as rolls), weir members for forming a melt receiver between said rolls and a tundish provided with a molten metal-pouring nozzle for pouring the molten steel into the melt receiver. The molten steel accumulated in the melt receiver is joined and pressed to be turned into a piece of slab by rotating the rolls and then the thin plate is produced by drawing out the slab.
However, when a surface of the molten steel poured into the melt receiver is oxidized or cooled, oxides, a shell formed on the surface thereof when cooled, dissolved substances from refractories of the tundish and the nozzle raised to the surface of the molten steel and the like are swallowed up by a flow of the molten steel. This causes internal defects or surface defects of the thin plate. Accordingly, the upper part of the surface of molten steel has been covered with a cover and inert gases have been poured into the cover to prevent the oxidation. Also a measure, in which a surface of molten steel is covered with heat-insulating powders to not only cut off air and prevent a temperature reduction but also adsorb the dissolved refractory substances thereby reducing a quantity of the dissolved refractory substances swallowed up by the flow of the molten metal, has been thought.
However, the above described conventional measure using inert gases has shown a problem in that a space within the cover has not only a large volume but also a large surface area, so that a radiant heat capacity is large thereby the surface of the molten steel is cooled to form the shell on the surface of the molten steel. In addition, in the case where the powders are used, if the powders are swallowed up by the flow of the molten steel to not only make the slab not uniform in thickness but also be turned into floating materials which are mixed in the molten steel, the internal defects are brought about, so that the defects are brought about in the products similarly.